you can also go into settings in the project window and go into the video display setting and set the prefill to 10 seconds (max I believe). this will load as many frames as possible into RAM before it attempts to play back and you have a better chance it will be able to deal with a more complex sequence without having to render first. If it is unable to load 10 seconds worth then it is due to a physical RAM limitation, i.e you dont have enough.
Otherwise expert render works well. right click on the timeline and do an expert render to render what it needs to in order to playback.
Video quality settings are a great, quick way to throw away frame information in order to playback without having to render first.
There will come a point when you have to render or do a video mix down in order to continue but you can get pretty complex before booking a ticket there.